Coulometer and timing apparatus

ABSTRACT

A reversible coulometer includes a pair of electrodes wherein each electrode alternates between functioning as an anode and as a cathode on alternate cycles. The coulometer includes a container and a liquid alkaline electrolyte in the container. A first electrode is in contact with the electrolyte and comprises an inert support and a solid active cadmium-containing material. A second control electrode comprises a liquid cadmium amalgam containing not more than about 1 percent cadmium by weight, the second electrode having an electrochemical energy storing capacity corresponding to about 1 percent of the capacity of the first electrode. A layer of dielectric material, permeable by the electrolyte and impermeable by the amalgam is in contact with the electrolyte and the amalgam to separate the electrodes. Means are provided for reversibly connecting the two electrodes in alternate sequence in a circuit which is connected to a source of current. During operation of the coulometer, the second electrode is alternately converted to substantially fully charged (reduced) and discharged (oxidized) conditions. The liquid amalgam maintains a supply of available, electrochemically active cadmium material at the liquid surface of the electrode on each anodic and cathodic cycle for the second electrode. The relatively small capacity of the second electrode relative to the first electrode then cooperates with the liquid surface characteristic of the second electrode so that the relative surface areas of the electrodes and the current density therebetween are maintained substantially constant from cycle to cycle. Also disclosed is electrical timing apparatus comprising the source of current, a resistor having a resistance which is a function of its temperature, and a current integrating device, such as the coulometer. Means are connected to the coulometer for signalling when the second electrode has reached a preselected charge or discharge level. Means are provided for interconnecting the current source and the resistor in a circuit with the reversible coulometer. Shunt-connected across the coulometer electrodes is a voltage limiter which limits the rise of voltage across the coulometer to a level below that at which hydrogen evolution will take place in the coulometer. The length of time for the signalling device to indicate the coulometer has reached a preselected charge or discharge level after reversing the sequence of connection of the electrodes with respect to the current source is a function of both time and temperature.

United States Patent Jest [54] COULOMETER AND TIMING APPARATUS [72]Inventor: Ernest M. Jost, Plainville, Mass. [73] Assignee: TexasInstruments Incorporated, Dallas,

Tex.

[22] Filed: Dec. 17, 1969 [21] Appl. No.: 885,785

[52] US. Cl ..324/l82, 324/94, 317/230 [51] Int. Cl. J...G04f 9/00, G0 1r 19/00 [58] Field of Search ..324/182, 94; 317/230 [56] ReferencesCited UNITED STATES PATENTS 3,428,894 2/1969 Boettcher ..324/943,302,091 1/1967 Henderson ..324/94 X 929,371 7/1909 Arsem ..3l7/2303,344,343 9/1967 John ..324/94 X Primary Examiner-Alfred E. SmithAttorney-Harold Levine, Edward J. Connors, Jr., John A. Haug, James P.McAndrews and Gerald B. Epstein [5 7] ABSTRACT A reversible coulometerincludes a pair of electrodes wherein each electrode alternates betweenfunctioning as an anode and as a cathode on alternate cycles. Thecoulometer includes a container and a liquid alkaline electrolyte in thecontainer. A first electrode is in contact with the electrolyte andcomprises an inert support and a solid active cadmium-containingmaterial. A second control electrode comprises a liquid cadmium amalgamcontaining not more than about 1 percent cad- [451 Jan. 25, 1972 mium byweight, the second electrode having an electrochemical energy storingcapacity corresponding to about 1 percent of the capacity of the firstelectrode. A layer of dielectric material, permeable by the electrolyteand impermeable by the amalgam is in contact with the electrolyte andthe amalgam to separate the electrodes. Means are provided forreversibly connecting the two electrodes in alternate sequence in acircuit which is connected to a source of current. During operation ofthe coulometer, the second electrode is alternately converted tosubstantially fully charged (reduced) and discharged (oxidized)conditions. The liquid amalgam maintains a supply of available,electrochemically active cadmium material at the liquid surface of theelectrode on each anodic and cathodic cycle for the second electrode.The relatively small capacity of the second electrode relative to thefirst electrode then cooperates with the liquid surface characteristicof the second electrode so that the relative surface areas of theelectrodes and the current density therebetwecn are maintainedsubstantially constant from cycle to cycle. Also disclosed is electricaltiming apparatus comprising the source of current, a resistor having aresistance which is a function of its temperature, and a currentintegrating device, such as the coulometer, Means are connected to thecoulometer for signalling when the second electrode has reached apreselected charge or discharge level. Means are provided forinterconnecting the current source and the resistor in a circuit withthe reversible coulometer. Shunt-connected across the coulometerelectrodes is a voltage limiter which limits the rise of voltage acrossthe coulometer to a level below that at which hydrogen evolution willtake place in the coulometer. The length of time for the signallingdevice to indicate the coulometer has reached a preselected charge ordischarge level after reversing the sequence of connection of theelectrodes with respect to the current source is a function of both timeand temperature.

16 Claims, 8 Drawing Figures COULOMETER AND TIMING APPARATUS Currentintegrating devices such as capacitors or coulometers have recognizedutility as timing devices. The principle utilized with respect to acoulometer is that the amount of chemical change produced by anelectrical current through an electrochemical cell is directlyproportional to the product of the magnitude of the current and the timefor which it is passed. In other words, the amount of chemical changeproduced by passage of an electrical current is directly proportional tothe quantity of electricity or coulombs passed. It is advantageous forthe current integrator of the timing device to be reversible and nothave to be reconditioned before it can be reused. The time required topass a given amount of change must be reproducible in either directionin order that the device may be used to measure time consistently andrepetitively.

Where time periods of a few seconds to a few minutes are to be measured,relatively high current densities are involved. Coulometers as describedin this invention are particularly desirable as current integratingdevices for timers of this type as the surface area of the controlelectrode remains constant in itself and remains constant relative tothe other electrode so that relative electrode surface area and currentdensity are not variable factors in the coulometer. Instead, themagnitude of the current flowing through the coulometer determines thelength of the period measured by the coulometer. Conventionalcoulometers are not able to handle the current densities required forsuch timing periods and permit rapid recycling. That is, if conventionalcoulometers are used in such timers and they are recycled (i.e., chargedrepetitively with alternate polarity connections), the relativeelectrode'surface areas and the current density between the electrodeswill change from cycle to cycle and inaccurate timing will result.

It would also be advantageous for certain important uses if such timerswould provide a timing period which is a function of temperature. Forexample, in the well-known Polaroid cameras the time required to developthe photographic print is a function of temperature and if the ambienttemperature is high a shorter development time is necessary. Thus, itwould be desirable for this and other uses to have a timer that providesa signal at the end of a predetermined time period under giventemperature conditions, but would automatically shorten or lengthen thistime period as a function of increased or decreased ambient temperature.

Among the several objects of this invention is the provision of areversible electrochemical coulometer which will handle relatively highcurrent densities for timing cycles of a few seconds to a few minutesand will accurately and reproducibly time such periods duringimmediately sequential repetitive alternate cycles and not driftfromcycle to cycle; the provision of electrodes for a reversiblecoulometer which provide a substantially constant current density fromcycle to cycle; the provision of timing apparatus wherein the periodtimed is a function of temperature; the provision of switch apparatusfor reversible coulometers and the like which provides a unidirectionalaction in alternately connecting a coulometer in opposite polaritymodes; and the provision of compact, economical and reliable timingapparatus. Other objects and features will be in part apparent and inpart pointed out hereinafter.

Briefly, the present invention is directed to a reversible coulometerwherein each electrode alternates between functioning as an anode and asa cathode on alternate cycles. The coulometer includes a container and aliquid alkaline electrolyte in the container. A first electrode is incontact with the electrolyte and comprises an inert support and a solidactive cadmium-containing material. A second control electrode isprovided which comprises a liquid cadmium amalgam containing not morethan about 1 percent cadmium. The second electrode preferably has anelectrochemical energy storing capacity corresponding to about 1 percentof the capacity of the first electrode. A layer of dielectric material,permeable by the electrolyte and impermeable by the amalgam, is incontact with the electrolyte and with the amalgam to separate theelectrodes. Means are provided for reversibly connecting the twoelectrodes in alternate sequence to a source of current whereby duringoperation of the coulometer the active material on the second electrodeis alternately converted to substantially fully charged (reduced) ordischarged (oxidized) condition. The liquid amalgam maintains a supplyof available electrochemically active cadmium material at the surfacethereof on each anode and cathode cycle for the second electrode. Therelative small capacity of the second electrode relative to the firstelectrode then cooperates with the surface characteristic of the secondelectrode so that the surface areas of the electrodes and currentdensity therebetween are maintained substantially constant from cycle tocycle.

Another aspect of this invention is electrical timing apparatusincluding a source of current, a resistor which has a resistance whichvaries as a function of its temperature, and a current integratingdevice such as the reversible coulometer. Means are connected to thecurrent integrating device for signalling when the current integratingdevice has reached a preselected charge or discharge level. Means areprovided for interconnecting the current source and the resistor in acircuit with the integrating device whereby the length of time for thesignalling device to indicate that the integrating device has reached apreselected charge or discharge level is a function of both time andtemperature.

In the accompanying drawings, in which several of various possibleembodiments of the invention are illustrated,

FIG. 1 is a longitudinal section of a reversible coulometer according tothis invention;

FIG. 2 is a longitudinal section of an alternate construction of areversible coulometer of this invention;

FIG. 3 is a section of another alternate construction of a reversiblecoulometer and switching apparatus therefor as taken on line 3-3 of FIG.5;

FIG. 4 is a bottom plan of a rotor of the FIG. 3 switching apparatus asviewed on line 4-4 of FIG. 3;

FIG. 5 is a section of the FIG. 3 reversible coulometer switchingapparatus taken on line 55 of FIG. 3;

FIG. 6 is a schematic diagram of an electrical timing apparatus usefulin the practice of this invention;

FIG. 7 is a schematic diagram of an alternate circuit of electricaltiming apparatus; and

FIG. 8 is a schematic diagram of another circuit of electrical timingapparatus.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

Referring now to the drawings, particularly FIG. 1, a reversiblecoulometer container of an electrically nonconductive material in theform of a hollow cylinder is indicated generally 'at referencenumeral 1. Extending from one end of container 1 is a lead wire orterminal connection 3 which is mechanically secured and electricallyconnected at its inner end to a first disk-shaped electrode 5. Thiselectrode is fabricated of a nickel wire mesh plaque to which nickelcarbonyl powder has been sintered, made, for example, in the mannerdisclosed in US. Pat. No. 3,393,096. The nickel plaque material is thenimmersed in cadmium nitrate solution to fill its pores and is placed ina hydroxide solution to precipitate cadmium hydroxide in situ within theplaque pores.

An O-ring 7 is seated on one side of electrode 5 to peripherally seal itin a coaxial position within the bore of the cylindric container 1.Electrode 5 is backed by a sealing plug or filler 9 of any conventionalpotting compound 9, such as formed from epoxy resins and the like. Acentral portion 11 of cylinder 1 necks inwardly to provide a reduceddiameter portion. An O-ring 8 is interposed between the other side ofelectrode 5 and a disk-shaped layer 13 of an electrolyte-permeabledielectric material thereby sealing the peripheral edges of bothrelative to each other and the shoulder formed at one side of thereduced diameter portion 11. A suitable dielectric material for layer 13is pressed cellulose fibers such as is commonly known as sausage casing.The space between electrode 5 and sausage casing layer 13 is filled withan alkaline electrolyte 15 such as potassium hydroxide. An aqueoussolution of 30 percent potassium hydroxide by weight or 6.9 Mconcentration is particularly effective as the electrolyte.

Reduced diameter portion 11 of container cylinder 1 is filled withlayers of another electrolyte-permeable dielectric material 17. Amaterial particularly useful for this is nylon felt such as is availableunder the trade designation Pellon." These layers are saturated withelectrolyte 15.

Another electrically conductive electrolyte-inert metallic support disk19 of nickel or cadmium-plated nickel, for example, is positioned withinthe bore of container 1 and peripherally sealed on both sides by twoother O-rings 21 and 23. The inner (or left as viewed in FIG. 1) surfaceof electrode disk 19 is spaced from another electrolyte-permeable layer25 of sausage casing and'sealed peripherally relative thereto by O-ring23. The space between the opposing surfaces of disk 19 and layer 25 isfilled with a body of a liquid cadmium amalgam 27. In an alternateconstruction within the scope of this invention, the body of liquidamalgam can be replaced with a thin film of the amalgam adhered to allsurfaces of an acrylic polymer disc or the like to facilitate retentionof the liquid amalgam between the disk 19 and the layer 25.

The amalgam desirably contains not more than 1 percent by weight ofcadmium and preferably from one-half to 1 percent by weight of cadmium.The cadmium used is 99.999 percent pure. The mercury is triple distilledfor high purity. The materials are combined by cutting a sheet of thecadmium into small pieces and mixing the small pieces with the mercuryin an inert atmosphere such as nitrogen. The cadmium particles arecompletely dissolved in the mercury without any difficulty to form atrue alloy. Disk 19 and the body of liquid cadmium amalgam 27 constitutethe second electrode of this coulometer 1.

The electrolyte-permeable, amalgam-impermeable, dielectric'maten'allayer 25 separates the amalgam 27 from electrolyte 15 in the centralportion 11 of the container 1 and also retains the amalgam 19 in surfacecontact with the support disk 17. Disk 19 is backed and sealed byfilling the remaining bore portion of container 1 with potting compound9. A lead wire or terminal connection 29 is connected to electrode disk17 and extends through the infilling of potting compound 9.

It is to be understood that the electrolyte-penneable layers 13, 17 and25 permit free migration of the hydroxyl ions while preventing thepassage of the amalgam, the cadmium hydroxide, and metal particles whichotherwise would tend to form dendrites during coulometer cycling. Thereversible electrochemical reaction which takes place at the cadmiumamalgam electrode is represented as:

Cd+2(OI'I)'- Cd(OI-I) +2e while the reaction at the cadmium hydroxideimpregnated electrode is represented as: i

Cd(0H) +2e Cd+2(OI-I)' When the current flow through the cell isreversed, the above reactions reverse.

The second electrode formed by the liquid amalgam 27 and the disk 19 arepreferably proportioned so that the second electrode has anelectrochemical energy storing capacity corresponding to about 1 percentof the electrochemical energy storing capacity of the first electrode 5.However, the second electrode can also have a much larger capacityrelative to the first electrode, up to about percent of the firstelectrode capacity, within the scope of this invention. The firstelectrode 5 is preferably provided with its active cadmium material inpartly charged (reduced) condition and in partly discharged (oxidized)condition, the preferred arrangement of the electrode 5 having aboutone-half of its active material in each of said conditions.

An alternate reversible coulometer construction is shown in FIG. 2wherein like numbers have been used for like parts. In the constructionshown here the layer 13 and O-rings 8 and 21 have been eliminated andelectrode disk 5 is seated directly against the left shoulder formed bythe reduced diameter central portion 11 of container 1. Similarly, thenylon felt layers 15 have been eliminated, the necked in central portion11 being filled with the same electrolyte 15 as before. Thisconstruction has also been found to be quite satisfactory.

Referring now to FIGS. 3-5, another embodiment of a reversiblecoulometer is indicated generally at numeral 31. This coulometer ismounted within the recessed end of a cylindrical rotor 33 of insulatingmaterial. This rotor is mounted for rotation around its central axiswithin an insulating base 35 provided with three identical resilientmetal contact fingers 37, 39 and 41 that have their free ends biased, asshown in FIG. 5, into wiping engagement with a lower portion 43 of rotor33 so that contact finger 37 contacts the lower rotor portion 43 at anarea 180 displaced from the lower rotor portion contacted by the ends ofcontact fingers 39 and 41. Inset into opposite surfaces of lower rotorportion 43 are two elongate metal contacts 45 and 47. As illustratedmore particularly in FIG. 4, the lower portion 43 has four ratchet teeth51 positioned at 90 intervals around its peripheral surface. The rotoris retained in base 35 by a machine screw 53 threaded into a threadedbore 55 of rotor 33. A pair of bearing washers 57 provide for rotationof rotor 33 within base 35.

The free ends of contact fingers 37, 39 and 41 also constitute pawlswhich permit rotation of rotor 33 in one direction (clockwise as shownin FIG. 5) but restrain rotation in the other direction. It is to beunderstood that other pawl means, separate from the contact fingers, maybe utilized instead of employing the contact fingers for the doublefunction of current carrying and providing pawl action relative to theratchet teeth 51.

Coulometer 31 has as its container a flanged metal cup 59, such as istypically used for transistors and other solid-state devices. A firstelectrode 61 of a liquid cadmium amalgam, as described previously, isretained in the upper portion of container 59 by a layer 63 ofelectrolyte-permeable, amalgam-impermeable dielectric material, such assausage casing. Immediately below layer 63 is a separator layer 65 offelted nylon, as described above, and which contains the alkalineelectrolyte 64. A second electrode 67 constituted, e.g., by a nickelmesh disk and a sintered active cadmium-containing material, similar toelectrode 5 of FIGS. 1 and 2, is held in position and sealed by pottingcompound 69. An insulating liner 71 of electrolyte-impermeabledielectric material isprovided around the inner surface of cup 59 toinsulate electrode 67 from cup 59 which is electrically common with theamalgam electrode 61. Conductive leads 73 and 75 interconnect thesecoulometer electrodes 67 and 61 with contacts 47 and 45 respectively.

The switch apparatus of FIGS. 3-5 has conductive leads 77, 79 and 81connected respectively to the supported ends of contact fingers 37, 39and 41. These leads provide for interconnection of coulometer 31 and theswitch apparatus with other circuit components of electrical timingapparatus of this invention as exemplified by the schematic circuitdiagrams of FIGS. 6-8. As illustrated more particularly in FIG. 6, suchtiming apparatus includes a battery 83 as a source of direct currentconnected with a thermistor 85 and the electrodes of coulometer 31 viathe switch apparatus by leads 77 and 79. The resistance of thermistor 85varies as a function of temperature, and for example, may have anegative thermal coefficient of resistivity if it is desired that theperiod being timed be shortened as a function of increasing temperature.

Also, series connected with battery 83 via leads 79 and 81 and theswitch apparatus is the collector-emitter circuit of a transistor 87 andan incandescent lamp 89 which serves as a means for signalling when thecoulometer has reached a preselected charge level. The base oftransistor 87 is connected by means of a diode 91 and lead 77 to contactfinger 37. Shunt connected across contact fingers 45 and 47 is aclamping diode 93 which serves as a means for limiting voltage developedacross coulometer 31. In the preferred timer construction illustrated,the transistor 87 and the diode 91 are of the germanium type while theclamping diode 93 is of the silicon type, these preferred componentshaving the voltage, current and junction characteristics to perfonntheir desired functions in the illustrated circuits at the voltagelevels provided by the reversible cadmium coulometer 31. However ifdesired, other diode and transistor types can also be used in thedescribed circuits if additional circuit components are employed inknown manner to modify the circuit characteristics within the scope ofthis invention.

When it is desired to initiate a timing cycle rotor 43 of the switchapparatus is moved to a position in which contact finger 37 contacts oneof the contacts 45 or 47 and the other contact fingers 39 and 41simultaneously contact the other contact. Current will then be suppliedto coulometer 31 and a charging cycle begins during which coulometer 31integrates the current supplied. Assuming typical conditions andparameters with battery 83 having a potential of 1.35 volts and with theliquid amalgam electrode in oxidized condition, the voltage across thecoulometer 31 will remain at a relatively low level, e.g., about 0.12volts until the liquid amalgam electrode is substantially fully reducedand the coulometer is substantially fully charged" (approximately thelast 5 percent of its cycle), whereupon the voltage across thecoulometer will sharply rise to a level of about 1.2 volts. However, aswill be discussed hereafter, this voltage rise is limited to about 0.6volt by the clamping action of diode 93. If the liquid amalgam electrodehad been in fully reduced condition at the initiation of the timingcycle, the voltage across the coulometer would have remained at the0.12-volt level until the liquid amalgam electrode had beensubstantially fully oxidized, at which time the voltage across thecoulometer would have risen toward the 1.2-volt level.

During the charging cycle, the low voltage across the coulometer 31 isinsufficient to overcome the potential drop on the base-emitter junctionof transistor 87 and the diode 91. Thus transistor 87 will not be biasedinto conductance until the voltage across coulometer 31 rises to about0.5 volt at which level diode 91 and the base-emitter junction of thetransistor 87 will become conductive and will turn on transistor 87,thereby energizing lamp 89. As the charging time of coulometer 31 isdetermined by the voltage of battery 83, the internal resistances of thebattery and coulometer, and the resistance of thermistor 85, any changein the temperature of thermistor 85 will vary the total resistance inthe circuit. Thus the time interval required to charge the coulometerand bias transistor 87 into conductance so as to energize lamp 89primarily is a function of the temperature of thermistor 85. Thelimiting of the coulometer voltage by clamping diode 93 prevents theevolution and buildup of hydrogen gas on the coulometer electrodes whichwould otherwise occur at higher voltages, such as over 1.0 volt, towhich the coulometer would rise unless limited. Such hydrogen evolutionwould render the coulometer cell irreversible. After the signallingdevice has been energized by the coulometers reaching its preselectedcharge level, the switch rotor 43 is moved 90 clockwise (FIG. 5) to anintermediate position whereupon the contact fingers 37, 39 and 41 areout of contact with contacts 45 and 47 and the latter two contactfingers are spaced and insulated from each other. This removes thecoulometer 31 from the circuit with battery 83 and also disconnects theemitter of transistor 87 from the coulometer and the battery. Anothertiming cycle can be initiated only by rotating the rotor 90 furtherclockwise (FIG. 5)'as the ratchet-pawl action permits onlyunidirectional action of the rotor. In this position the coulometer isconnected in an opposite polarity mode relative to that previouslydescribed and shown in FIGS. 36. The electrochemical reactions at theelectrodes are then reversed as discussed above and another timing cycletakes place. Further 90 rotation of rotor 43 to a second intermediateposition again opens the circuits between the battery, coulometer andtransistor emitter. Thus, as the rotor of this unidirectional switch isrotated it moves from a first closed circuit position to an open circuitposition and then to a second closed circuit position in which thecoulometer is connected to the battery in a polarity mode opposite fromthat of the first closed circuit position, and then finally to a secondopen circuit position. A rapid sequence of repetitive timing cycles canbe carried out and because of the substantially constant current densitycharacteristics of this coulometer cell the accuracy and reproducibilityof the timed intervals are excellent. As this switching apparatus andcoulometer are compact, and the switching action is unidirectional andprovides rapid sequencing of timing cycles, they may be incorporatedadvantageously in a Polaroid camera with the rotor linked or geared tothe film pullout rollers and mechanism to time the film developmentperiods, automatically compensating for temperature variations andsupplying an indication signalling the end of each suchtemperature-compensated time period.

An exemplary timing apparatus such as described has the followingcharacteristics and parameters: The diameter of coulometer 31 is 0.8 cm.with the sintered counter electrode 67 having approximately 25 ma. hr.capacity. The cadmiumamalgam electrode 61 has approximately a 0.25 ma.hr. capacity and remains liquid over the usual ranges of ambienttemperatures. The current densities for l-minute timing cycles are onthe order of 30 ma./cm. on electrodes 61 and 67. This amalgam electrode67 provides a signal level of at least 0.5 v. on both the hydrogen andoxygen cycles.

Presently available thermistors have a thermal coefficient ofresistivity such that typically the maximum change in resistance perdegree temperature change is about 4 percent. As there may be fields ofuse where this timing apparatus desirably should have a greater orenhanced sensitivity or response to temperature change, two furthertiming apparatus circuits are disclosed in FIGS. 7 and 8 which provideincreased values of resistance change per degree temperature variation.

The FIG. 7 circuit is similar to that of FIG. 6 except that a transistor95 has its output or collector-emitter circuitconnected in the chargingcurrent circuit in place of the resistance of thermistor 85. Thisthermistor and a resistor 97 are series connected across thecollector-emitter circuit of transistor 95. The base of this transistoris connected to the junction between thermistor and resistor 97. Anytemperature-induced variation in the resistance of thermistor 85 willchange the bias and vary the conductivity of transistor 95, thuschanging the resistance of the circuit interconnecting the battery 83and coulometer 31 at a greater rate per degree temperature change thanthat of thermistor 85.

Another means for increasing the thermal response of the timingapparatus is illustrated in FIG. 8. Another thermistor 99, also ofnegative temperature coefficient where the thermistor 85 is of negativetemperature coefficient, is series connected in the coulometer chargingcircuit with thermistor 85. Connected between the junction of thesethermistors and the negative terminal of battery 83 is a resistor 101.Thermistor 99 and resistor 101 constitute a voltage divider connectedacross battery 83 with the voltage at the junction therebetween being afunction of the rate of thermally induced change of the resistance ofthermistor 99. When temperature increase occurs, voltage at the junctionbetween the thermistors 99 and 85 increases due to lowering of theresistance in the upper half of the voltage divider. The potential dropbetween the junction of the thermistors and the junction betweenthermistor 85 and the diode 93 then also increases directing increasingcurrent through thermistor 85. At the same time, thermistor 85 decreasesin resistance in response to the same temperature change which effectedthermistor 99. The combined change in resistance in the coulometercircuit in response to the temperature change is thus much greater withthe two thermistors arranged in this manner than could be achieved witheither thermistor alone.

It will be understood as discussed above that although coulometers havecertain advantages over capacitors as current integrating devices in thetiming apparatus of this invention, capacitors may be used for thispurpose. Also, it should be noted that signalling means other than anelectrically energized light source may be advantageously used, e.g., anelectromagnetic device such as a relay or solenoid which is energizedwhen the current integrating device reaches its preselected chargelevel.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes can be made in the above constructions, withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

1. A reversible coulometer wherein each electrode alternates betweenfunctioning as anode and cathode on alternate cycles which comprises:

a container;

a liquid alkaline electrolyte in the container;

a first electrode in contact with the electrolyte and comprising aninert support and a solid active cadmium-containing material, said firstelectrode being in partly charged and partly discharged condition;

a second electrode in contact with the electrolyte comprising a liquidcadmium amalgam containing not more than about 1 percent cadmium byweight, said second elecv trode having a relatively smallerelectrochemical energy storing capacity than said first electrode;

a layer of dielectric material, permeable by the electrolyte andimpermeable by the amalgam, in contact with the electrolyte andseparating the electrodes; and

means reversibly connecting the two electrodes to a source of currentwhereby during operation of the coulometer the liquid amalgam maintainsa supply of available cadmium material at the surface thereof on eachcycle for the second electrode so that the surface area of theelectrodes and the current density are maintained substantially constantfrom cycle to cycle.

2. A reversible coulometer as set forth in claim 1 wherein said firstelectrode is in about one-half charged condition and said secondelectrode has an electrochemical energy storing capacity correspondingto about 1 percent to 20 percent of the electrochemical energy storingcapacity of said first electrode.

3. A coulometer as set forth in claim 2 wherein said liquid amalgam isadhered to all surfaces of an acrylic polymer disk.

4. A coulometer of claim 1 wherein the container comprises a memberhaving a cylindrical chamber, the first electrode comprises a disksealed in and coaxial with said cylindrical chamber, the secondelectrode comprises a relatively thin layer of said liquid cadmiumamalgam sealed in said cylindrical chamber, and wherein said layer ofdielectric 'material comprises at least one disk of said materialdisposed between said electrodes.

5. A coulometer of claim 1 wherein the layer of dielectric materialcomprises pressed cellulose fibers.

6. A coulometer of claim 1 wherein the electrolyte is a potassiumhydroxide solution.

7. A coulometer of claim 6 wherein the electrolyte contains 30 percentpotassium hydroxide by weight.

8. A coulometer of claim 1 wherein the first electrode comprises aporous plaque of nickel carbonyl powder impregnated with cadmiumhydroxide and sintered to a nickel wire mesh.

9. In a reversible coulometer, the improvement which comprises a firstelectrode having solid active cadmium-containing material and a secondliquid cadmium-amalgam electrode containing not more than about 1percent cadmium by weight, said first electrode having a relativelylarger electrochemical energy storing capacity than said secondelectrode and being in partly charged and partly discharged condition,whereby during operation of the cell the second electrode maintains asupply of available cadmium material at the surface thereof on eachcycle for the electrode so that the surface area of the electrodes andthe current density remain substantially constant from cycle to cycle.

10. Electrical timing apparatus compnsmg:

a source of direct current;

a resistor which has a resistance which varies as a function of itstemperature;

a coulometer comprising a container; a liquid alkaline electrolyte inthe container; a first electrode in contact with the electrolyte andcomprising an inert support and a solid active cadmium-containingmaterial, said first elec trode being in partly charged and partlydischarged con dition; a second electrode in contact with theelectrolyte comprising a liquid cadmium-amalgam containing not more thanabout 1 percent cadmium by weight, said second electrode having arelatively smaller electrochemical energy storing capacity than saidfirst electrode; a layer of dielectric material, permeable by theelectrolyte and impermeable by the amalgam, in contact with theelectrolyte and separating the electrodes;

means connected to the coulometer for signalling when the coulometer hasreached a preselected charge level; and means for interconnecting saidcurrent source and said resistor in a circuit with said coulometerwhereby the length of time for the signalling device to indicate thecoulometer has reached a preselected charge level is a function of bothtime and temperature.

11. Timing apparatus of claim 10 including polarity reversing means forresetting the coulometer after the desired charge level has been reachedwhereby another timing cycle may be carried out.

12. Timing apparatus of claim 11 wherein said second electrode has anelectrochemical energy storing capacity corresponding to about 1 percentto 20 percent of the electrochemical energy storing capacity of saidfirst electrode.

13. Timing apparatus of claim 11 wherein said liquid amalgam is adheredto all surfaces of an acrylic polymer disk.

14. Timing apparatus of claim 13 wherein the means for interconnectingthe current source and resistor with said coulometer includes a rotaryswitch having two open circuit positions alternating with two closedcircuit positions, said first and second closed circuit positionsrespectively interconnecting the current source through said resistor tosaid coulometer in opposite polarity modes, said switching includingmeans for permitting actuation of said switch in one rotationaldirection only.

15. Timing apparatus of claim 14 wherein the resistor has a negativethermal coefficient of resistivity.

l6. Timing apparatus of claim 10 wherein the means for interconnectingthe current source and resistor with said coulometer includes switchingmeans having at least one open circuit position and two closed circuitposition said first and second closed circuit positions respectivelyinterconnecting the current source through said resistor to saidcoulometer in opposite polarity modes.

* a s: a

1. A reversible coulometer wherein each electrode alternates betweenfunctioning as anode and cathode on alternate cycles which comprises: acontainer; a liquid alkaline electrolyte in the container; a firstelectrode in contact with the electrolyte and comprising an inertsupport and a solid active cadmium-containing material, said firstelectrode being in partly charged and partly discharged condition; asecond electrode in contact with the electrolyte comprising a liquidcadmium amalgam containing not more than about 1 percent cadmium byweight, said second electrode having a relatively smallerelectrochemical energy storing capacity than said first electrode; alayer of dielectric material, permeable by the electrolyte andimpermeable by the amalgam, in contact with the electrolyte andseparating the electrodes; and means reversibly connecting the twoelectrodes to a source of current whereby during operation of thecoulometer the liquid amalgam maintains a supply of available cadmiummaterial at the surface thereof on each cycle for the second electrodeso that the surface area of the electrodes and the current density aremaintained substantially constant from cycle to cycle.
 2. A reversiblecoulometer as set forth in claim 1 wherein said first electrode is inabout one-half charged condition and said second electrode has anelectrochemical energy storing capacity corresponding to about 1 percentto 20 percent of the electrochemical energy storing capacity of saidfirst electrode.
 3. A coulometer as set forth in claim 2 wherein saidliquid amalgam is adhered to all surfaces of an acrylic polymer disk. 4.A coulometer of claim 1 wherein the container comprises a member havinga cylindrical chamber, the first electrode comprises a disk sealed inand coaxial with said cylindrical chamber, the second electrodecomprises a relatively thin layer of said liquid cadmium amalgam sealedin said cylindrical chamber, and wherein said layer of dielectricmaterial comprises at least one disk of said material disposed betweensaid electrodes.
 5. A coulometer of claim 1 wherein the layer ofdielectric material comprises pressed cellulose fibeRs.
 6. A coulometerof claim 1 wherein the electrolyte is a potassium hydroxide solution. 7.A coulometer of claim 6 wherein the electrolyte contains 30 percentpotassium hydroxide by weight.
 8. A coulometer of claim 1 wherein thefirst electrode comprises a porous plaque of nickel carbonyl powderimpregnated with cadmium hydroxide and sintered to a nickel wire mesh.9. In a reversible coulometer, the improvement which comprises a firstelectrode having solid active cadmium-containing material and a secondliquid cadmium-amalgam electrode containing not more than about 1percent cadmium by weight, said first electrode having a relativelylarger electrochemical energy storing capacity than said secondelectrode and being in partly charged and partly discharged condition,whereby during operation of the cell the second electrode maintains asupply of available cadmium material at the surface thereof on eachcycle for the electrode so that the surface area of the electrodes andthe current density remain substantially constant from cycle to cycle.10. Electrical timing apparatus comprising: a source of direct current;a resistor which has a resistance which varies as a function of itstemperature; a coulometer comprising a container; a liquid alkalineelectrolyte in the container; a first electrode in contact with theelectrolyte and comprising an inert support and a solid activecadmium-containing material, said first electrode being in partlycharged and partly discharged condition; a second electrode in contactwith the electrolyte comprising a liquid cadmium-amalgam containing notmore than about 1 percent cadmium by weight, said second electrodehaving a relatively smaller electrochemical energy storing capacity thansaid first electrode; a layer of dielectric material, permeable by theelectrolyte and impermeable by the amalgam, in contact with theelectrolyte and separating the electrodes; means connected to thecoulometer for signalling when the coulometer has reached a preselectedcharge level; and means for interconnecting said current source and saidresistor in a circuit with said coulometer whereby the length of timefor the signalling device to indicate the coulometer has reached apreselected charge level is a function of both time and temperature. 11.Timing apparatus of claim 10 including polarity reversing means forresetting the coulometer after the desired charge level has been reachedwhereby another timing cycle may be carried out.
 12. Timing apparatus ofclaim 11 wherein said second electrode has an electrochemical energystoring capacity corresponding to about 1 percent to 20 percent of theelectrochemical energy storing capacity of said first electrode. 13.Timing apparatus of claim 11 wherein said liquid amalgam is adhered toall surfaces of an acrylic polymer disk.
 14. Timing apparatus of claim13 wherein the means for interconnecting the current source and resistorwith said coulometer includes a rotary switch having two open circuitpositions alternating with two closed circuit positions, said first andsecond closed circuit positions respectively interconnecting the currentsource through said resistor to said coulometer in opposite polaritymodes, said switching including means for permitting actuation of saidswitch in one rotational direction only.
 15. Timing apparatus of claim14 wherein the resistor has a negative thermal coefficient ofresistivity.
 16. Timing apparatus of claim 10 wherein the means forinterconnecting the current source and resistor with said coulometerincludes switching means having at least one open circuit position andtwo closed circuit position said first and second closed circuitpositions respectively interconnecting the current source through saidresistor to said coulometer in opposite polarity modes.